In-line fiber optic probes have been used for identifying, or for monitoring the parameters of, a fluid flowing in a sample, recirculating or a process line. Examples of such probes are found in U.S. Pat. Nos. 4,573,761 and 4,707,134, incorporated herein by reference. An improved probe of this type is described in pending U.S. Pat. application Ser. No. 07/236,571, entitled "Sealed Fiber Optic Probe", filed Aug. 25, 1988, also incorporated herein by reference. These disclosures are not essential to an understanding or description of the invention of this application, but are included for background purposes.
In sampling fluid by spectrophotometry, one typical practice has been to mount two fiber optic probes such that their ends face each other across a gap. Fluid flows in this gap while light from one probe is directed through the fluid to the other probe. The light received is analyzed by known spectrophotometry processes to provide identification of the fluid or monitoring of selected parameters thereof.
Typically, the probes are mounted in a tubular body or cell which also has a fluid inlet, a fluid outlet and a fluid passageway extending through the gap between the probe ends. One such device is an in-line absorbance cell manufactured by Guided Wave Inc., of El Dorado Hills, Calif. In that device, fiber optic probes are mounted by means of compression ferrule seal fittings mounted in the opposite ends of a tube. Such fittings include ferrule and compression members which interact to engage the probes. The ends of the probes are spaced apart and a fluid inlet and outlet are disposed in the tube sidewall for passing fluid through the tube and between the probe ends.
The consistency of optical alignment of each probe in such a device is poor, due mainly to the large tolerance ranges produced by the compression ferrule seal fittings, the relatively loose fit of probe to fitting and the use of diverse fitting materials. These fittings tend to establish a pivot point or axis about which the probe moves or wanders away from its desired alignment when the device is adjusted or subjected to pressure or temperature changes.
In such forms of probe mounting, fitting threads according to National Pipe Thread standards allow a sloppy fit for pipe and adapter matings to be sealed with pipe dope. Such designs allow significant tolerance build-up, producing inconsistent optical alignment of an associated probe. Also, depending on the fluids to be tested, a sampling system may require exotic fitting and seal materials which are very expensive and require long lead times to produce.
Moreover, the different coefficients of expansion and the rigidity of diverse materials used in the same system generate further optical misalignment when the system is pressured or temperatures change. Such pressure and temperature changes can cause different material responses and can flex such a system, causing significant optical misalignment.
It should be appreciated that proper optical alignment of probes is a critical consideration whether probes are mounted on a common co-axial axis, as in the aforementioned in-line absorbance cell, or are mounted on respective optical intersecting axes at some predetermined angular disposition, such as in certain fluorescent, scattering or other techniques. If the probes in any such optical systems are misaligned, system efficiency is reduced. Moreover, shifts in alignment due to temperature or pressure changes are seen falsely by the system as fluid or parameter fluctuations.
Accordingly, it has been one objective of the invention to provide an in-line fiber optic probe interface with improved optical alignment of each probe, whether coaxially or otherwise aligned.